The present invention relates to optical waveguide grating devices and, in particular, to a waveguide grating device wherein the optical pathlength between successive grating elements (hereinafter xe2x80x9coptical spacingxe2x80x9d) can be adjusted with distance and time.
Optical waveguide gratings with adjustable optical spacing profiles are potentially valuable components in optical communication systems. Waveguide Bragg gratings can provide wavelength-dependent dispersion compensation. If the Bragg grating optical spacing is adjustable with distance and time, the grating can dynamically respond to changing spectral profiles of needed dispersion compensation. Long-period waveguide gratings can provide wavelength-dependent loss. If the long-period grating spacing is adjustable, the grating can dynamically respond to changing profiles of needed loss. Such adjustable gratings are of particular importance for contemplated broad band WDM systems where dynamic dispersion and amplitude compensation will be required.
An optical communication system comprises, in essence, a source of information-carrying optical signals, a length of optical waveguide for carrying the optical signals and a receiver for detecting the optical signals and demodulating the information they carry. Optical amplifiers are typically located along the waveguide at regular intervals, and add/drop nodes are disposed at suitable locations for adding and dropping signal channels. Conventional systems are usually based on high purity silica optical fiber waveguide and erbium-doped optical fiber amplifiers (EDFAs). Such systems introduce small differences in the propagation time and transmitted power of different wavelength signal components. For example longer wavelength components are subject to slightly longer delay than shorter wavelength components (chromatic dispersion) and wavelength components off the amplification peak of EDFAs will be transmitted with slightly less power. These phenomena can distort a transmitted pulse and limit bandwidth and/or transmission distance.
The performance of high speed WDM systems will depend on the ability of the system to compensate dispersion and wavelength dependent power fluctuations. Moreover in high speed systems, dynamic fluctuations within the system will change the spectral profile of required dispersion and power compensation as a function of time, making it very difficult to provide needed compensation with static devices.
A typical Bragg grating comprises a length of optical waveguide, such as optical fiber, including a plurality of perturbations in the index of refraction substantially equally spaced along the waveguide length. These perturbations selectively reflect light of wavelength X equal to twice the spacing A between successive perturbations times the effective refractive index, i.e. xcex=2 neffxcex9, where xcex is the vacuum wavelength and neff is the effective refractive index of the propagating mode. The remaining wavelengths pass essentially unimpeded. If the geometric spacing between successive perturbations changes as a function of distance into the grating (the grating is xe2x80x9cchirpedxe2x80x9d) different wavelengths will travel different distances into the grating before they are reflected. Thus chirped gratings provide different propagation delays to different wavelengths, and their geometric spacings can be chosen to compensate the components of a dispersed signal, i.e. the spacing can be chosen so that all spectral components receive the same total delay (See F. Ouellette, xe2x80x9cDispersion cancellation using linearly chirped Bragg filters in optical waveguidesxe2x80x9d, 12 Optics Letters 847-849 (1987)).
A typical long-period grating couples optical power between two copropagating modes with very low back reflections. It comprises a length of optical waveguide wherein a plurality of refractive index perturbations are spaced along the waveguide by a periodic distance xcex9xe2x80x2 which is large compared to the wavelength xcex of the transmitted light. In contrast with conventional Bragg gratings, long-period gratings use a periodic geometric spacing xcex9xe2x80x2 which is typically at least 10 times larger than the transmitted wavelength, i.e. xcex9xe2x80x2xe2x89xa710xcex. Typically xcex9xe2x80x2 is in the range 15-1500 micrometers, and the width of a perturbation is in the range 1/5 xcex9xe2x80x2 to 4/5 xcex9xe2x80x2. In some applications, such as chirped gratings, the spacing xcex9xe2x80x2 can vary along the length of the grating. Long-period gratings are particularly useful for equalizing amplifier gain at different wavelengths of an optical communications system. See, for example, U.S. Pat. No. 5,430,817 issued to A. M. Vengsarkar on Jul. 4, 1995, which is incorporated herein by reference.
A shortcoming of waveguide gratings as dispersion and power spectrum compensation devices is that they are permanent and narrow band. The spacing between successive perturbations is fixed in manufacture, fixing the compensating characteristics of the grating. High speed systems, however, require dynamic compensation.
One approach to providing waveguide gratings capable of dynamic compensation is to provide a plurality of electrical heaters along the length of the grating. Each heater is independently controlled to adjust the portion of the grating local to the heater by heating the waveguide material. Such heating thermally expands the material to change the geometric spacing between perturbations and also changes the index of refraction. It thus changes the optical path length between perturbations. The difficulty with this approach is that it requires many tiny heaters and many tiny connections and controls. Failure of any one heater connection along the sequence can be serious.
Another approach is set forth in copending application Ser. No. 09/183,048 filed by B. J. Eggleton et al. on Oct. 30, 1998 and entitled xe2x80x9cOptical Grating Devices With Adjustable Chirpxe2x80x9d, which is incorporated herein by reference. Here the grating is provided with a film of linearly varying electrical resistance. Application of a current to the film generates a linearly varying amount of heat along the length of the grating. A second uniform resistance film may be separately controlled to vary the average heat generated. This approach works well in simple systems where the needed compensation is of a known spectral slope. But because the heat generation is monotonic with distance, the device does not adjust well to systems where even the spectral slope of needed compensation can change. Accordingly there is a need for improved optical waveguide grating device wherein the optical pathlength between successive grating elements can be adjusted with distance and time.
In accordance with the invention, an optical waveguide grating with an adjustable optical spacing profile comprises a waveguide grating in thermal contact with one or more resistive film coatings. A coating extends along the length of the grating and its local resistance varies along the length of the grating. In one embodiment, a plurality of overlaping coatings are chosen so the resistance variation of each is different, thereby permitting a variety of heat generation profiles to be effected by independent control of the coatings. The different heat generation profiles, in turn, proportionately change the grating geometric spacing and local refractive index along the grating length, providing the desired adjustable optical spacing profile. Other embodiments use resistive films with abruptly changing or periodically changing heating variation.